Fran Gundrum (1856-1919) was a Croatian physician, encyclopedist, and an advocate of medical enlightenment and healthy lifestyle. In order to identify and analyze Gundrum’s ideas about the problems of prostitution and criminality, we studied all of his books, booklets, and articles published between 1905 and 1914. We showed that Gundrum’s theories of heredity, morality, and sexual hygiene incorporated many of the important discussions of his time, especially those related to the Darwinian paradigm. Gundrum’s project of collecting statistics on prostitutes was the first such study published on the territory of today's Croatia. Although he rejected the notions of born prostitutes and born criminals, defended by Italian criminal anthropologist Cesare Lombroso, he still regarded eugenics as a convenient method of dealing with the ills of society. He believed that criminals were degenerate individuals representing a violent threat to the society and that it was legitimate to use radical means, such as sterilization and deportation, to deal with this problem. Organicistic view of the society prevented him from seeing the individual rights as important as that of the society to protect itself. Nevertheless, this view led to many humanistic ideas, such as the binomial illness/poverty in case of prostitution, which influenced many prominent works of social medicine movement.